1. Field of the Invention
This invention relates to optical fibers for use in optical fiber amplifiers and more particularly to phosphorus-silicate fibers.
2. Technical Background
Most of the current erbium doped fiber amplifiers (EDFAs) operate in the conventional band (C-band), approximately from about 1525 nm to about 1565 nm. Driven by the rapid growth in Internet, Metro and LAN applications, the wavelength division multiplexing (WDM) optical transmission systems employing EDFAs have to cope with the increasingly large capacity demands. Thus, it is important to develop new EDFA""s that not only provide a flat-gain shape, but also have the widest possible bandwidth.
Er-doped Alxe2x80x94Ge-Silica (Erxe2x80x94Alxe2x80x94Gexe2x80x94Si) fibers have been a common choice for use in EDFA""s operating in the long wavelength band (L-band) from 1565 nm to 1605 nm. However, in order to utilize the 1565 nm to 1620 nm capacity offered by the commercially available transmission fibers, there is a need to further increase EDFA""s signal band capacity to 1620 nm and beyond, a range known as the extended L-Band.
An article entitled xe2x80x9cOptical Amplification Characteristics around 1.58 xcexcm of Silica-Based Erbium-Doped Fibers Containing Phosphorous/Alumina as Codopantsxe2x80x9d published in the Technical Digest, Optical Amplifiers and their Applications Conference, 1998, describes alumina- phosphorus-silicate fibers. This article discloses that these fibers can be utilized in the L-band range. Table 1 of the article describes the core composition of three different fiber types: A, B, and C with varying concentrations of alumina and phosphorus. Type A fiber contains about 7.8 weight percent (wt %) alumina and no phosphorus. Type B fiber contains about 2 wt % alumina and 5 wt % phosphorus oxide. Type C fiber contains aluminum concentration of 0.3 wt % and phosphorus oxide concentration around 8 wt %. FIG. 3 of this article depicts the gain curve for each of those fibers. The gain curve for the Type B fiber is lower than that of Type A fiber, primarily because of lower Er concentration. However, although the amount of Er is identical in Type A and C fibers, the Type C fiber extends the L-band gain to longer wavelengths, relative to that of the Type A fiber. Type C fiber, however, provides smaller gain amount and has more gain ripple. Finally, because of its core/clad composition, the Type C fiber would have a relatively low refractive index delta between the core and the cladding (xcex94N less than 0.004), which results in low pump efficiency and high bending sensitivity. This would, in turn, result in a very large size module, due to high total power and a large coil diameter, and would make the amplifier commercially impractical.
The strong presence of AlPO4 in making silica-based Alxe2x80x94Pxe2x80x94Si fibers such as fibers Types B and C has been shown to be problematic. The AlPO4 units tend to cluster away from the silica-based structural network and form microcrystals (with typical grain sizes of less than 100 xcexcm). These microcrystals cause high scattering loss in the resultant fibers. Moreover, the AlPO4 units have a lower refractive index than those fibers with P2O5 and Al2O3. A relatively high xcex94N of about 1% is commonly desired for fibers utilized in EDFA applications. More alumina and phosphorus would be desirable to raise the refractive index of the core relative to the clad and to obtain optimal gain shape in the output of EDFA. However, elevated Al and P levels lead to the formation of a high concentration of AlPO4 units in the glass, which in turn result in lower xcex94N, and further aggravate the clustering and the resulting scattering problem.
An article entitled xe2x80x9cFabrication and Characterization of Yb3+:Er3+ Phosphosilicate Fibers for Lasersxe2x80x9d published in Journal of Lightwave technology, Vol. 16, No. 11, November 1998 also discloses optical fibers with optically active glass. This article, however, is directed to high power fiber lasers operating at 1.5 xcexcm, and specifically to Yb and Er co-doped fiber lasers. The article teaches that in order to achieve high output power a 1064 nm Nd: YAG laser is used to activate Yb elements, thus indirectly pumping Er ions. The excited Yb elements transfer energy to the Er ions, enabling optical signal amplification from the Er ions. More specifically, this article discloses that a high power (800 mW) Nd:YAG laser was used to achieve the high output power from the optical fiber. In a commercially deployable amplifier such a Nd:YAG pump laser would be prohibitively large. Therefore, a Nd:YAG laser would not be utilized in a typical optical amplifier where component miniaturization and space conservation are extremely important. Furthermore, such a laser can not be used effectively to directly pump Er ions.
According to one aspect of the present invention an optically active phosporus-silicate glass when pumped to directly excite Er ions, provides gain in 1565 nm to 1620 nm range and comprises in weight percent:
SiO2 50 to 92%;
Er2O3 0.01 to 2%;
P2O5 greater than 5%; and
Al2O3 0.0 to 0.3%.
According to an embodiment of the present invention this glass includes, in weight percent: SiO2 65 to 92%; Er2O3 0.01 to 1%; P2O5 greater than 5%; Al2O3 0 to 0.3%; and one or more oxides of the following elements: Ge, Yb; Y; Ga; Ta, Gd, Lu, La, and Sc in an amount from 0.1% to 20% wt %.
An advantage of the inventive glass is that it can be used in optical gain medium fiber in L-band optical amplifiers, extending the L-band beyond 1605 nm to 1620 nm or beyond.